National Security Agency leaker Edward Snowden has offered to help Brazil investigate US spying on its soil in exchange for political asylum, in an open letter to the Brazilian people published by a Brazilian newspaper.

Snowden wrote in a lengthy letter that he's been inspired by the global debate ignited by his release of thousands of NSA documents, and that the NSA's culture of indiscriminate global espionage "is collapsing."

In the letter, published by the Folha de S. Paulo newspaper and released widely online, Snowden commended the Brazilian government for its strong stand against US spying.

He said he'd be willing to help the South American nation investigate espionage on its soil, but could not fully participate in doing so without being granted political asylum.

"Until a country grants permanent political asylum, the US government will continue to interfere with my ability to speak," he says.

Revelations about the NSA's spy programmes were first published in June, based on some of the thousands of documents Snowden handed over to the Brazil-based American journalist Glenn Greenwald.

The documents revealed that Brazil is the top NSA target in Latin America, spying that has included the monitoring of Brazilian President Dilma Rousseff's mobile phone and hacking into the internal network of state-run oil company Petrobras.

The revelations enraged Rousseff, who in October cancelled an official visit to Washington that was to include a state dinner. She's also pushing the United Nations to give citizens more protection against spying.

Snowden dismissed US explanations that the bulk metadata gathered on billions of e-mails and calls was more "data collection" than surveillance.

"There is a huge difference between legal programmes, legitimate spying ... and these programmes of dragnet mass surveillance that put entire populations under an all-seeing eye and save copies forever," he wrote. "These programmes were never about terrorism: they're about economic spying, social control, and diplomatic manipulation. They're about power."

"Now, the whole world is listening back, and speaking out, too," Snowden wrote.

"The culture of indiscriminate worldwide surveillance, exposed to public debates and real investigations on every continent, is collapsing."

Brazilian senators have asked for Snowden's help during hearings about the NSA's targeting of Brazil, an important transit hub for trans-Atlantic fibre optic cables that are hacked. Both Greenwald and his domestic partner David Miranda spoke before the senate, and Miranda has taken up the cause of persuading Brasilia to grant political asylum to Snowden.

Snowden, who is living in Russia on a temporary one-year visa, previously requested political asylum in Brazil and several other nations.

Neither Brazil's foreign ministry nor the presidential office said they had immediate comment on Snowden's letter or any pending asylum request.

Several members of Brazil's congress have called for Snowden to receive asylum, so that he could assist lawmakers' investigation into NSA activity in Brazil.

Rousseff has also ordered her government to take several measures, including laying fibre optic lines directly to Europe and South American nations, in an effort to "divorce" Brazil from the US-centric backbone of the internet that experts say has facilitated NSA spying.

Additional reporting by Agence France-Presse

This article appeared in the South China Morning Post print edition as Snowden offers to help Brazil